README.md

Logrus

Logrus is a structured logger for Go (golang), completely API compatible with
the standard library logger.

Seeing weird case-sensitive problems? It's in the past been possible to
import Logrus as both upper- and lower-case. Due to the Go package environment,
this caused issues in the community and we needed a standard. Some environments
experienced problems with the upper-case variant, so the lower-case was decided.
Everything using logrus will need to use the lower-case:
github.com/sirupsen/logrus. Any package that isn't, should be changed.

Are you interested in assisting in maintaining Logrus? Currently I have a
lot of obligations, and I am unable to provide Logrus with the maintainership it
needs. If you'd like to help, please reach out to me at simon at author's username dot com.

Nicely color-coded in development (when a TTY is attached, otherwise just
plain text):

With log.SetFormatter(&log.JSONFormatter{}), for easy parsing by logstash
or Splunk:

Note that this does add measurable overhead - the cost will depend on the version of Go, but is
between 20 and 40% in recent tests with 1.6 and 1.7. You can validate this in your
environment via benchmarks:

go test -bench=.*CallerTracing

Case-sensitivity

The organization's name was changed to lower-case--and this will not be changed
back. If you are getting import conflicts due to case sensitivity, please use
the lower-case import: github.com/sirupsen/logrus.

Example

The simplest way to use Logrus is simply the package-level exported logger:

Note that it's completely api-compatible with the stdlib logger, so you can
replace your log imports everywhere with log "github.com/sirupsen/logrus"
and you'll now have the flexibility of Logrus. You can customize it all you
want:

package main
import (
"os"
log "github.com/sirupsen/logrus"
)
funcinit() {
// Log as JSON instead of the default ASCII formatter.
log.SetFormatter(&log.JSONFormatter{})
// Output to stdout instead of the default stderr// Can be any io.Writer, see below for File example
log.SetOutput(os.Stdout)
// Only log the warning severity or above.
log.SetLevel(log.WarnLevel)
}
funcmain() {
log.WithFields(log.Fields{
"animal": "walrus",
"size": 10,
}).Info("A group of walrus emerges from the ocean")
log.WithFields(log.Fields{
"omg": true,
"number": 122,
}).Warn("The group's number increased tremendously!")
log.WithFields(log.Fields{
"omg": true,
"number": 100,
}).Fatal("The ice breaks!")
// A common pattern is to re-use fields between logging statements by re-using// the logrus.Entry returned from WithFields()contextLogger:= log.WithFields(log.Fields{
"common": "this is a common field",
"other": "I also should be logged always",
})
contextLogger.Info("I'll be logged with common and other field")
contextLogger.Info("Me too")
}

For more advanced usage such as logging to multiple locations from the same
application, you can also create an instance of the logrus Logger:

package main
import (
"os""github.com/sirupsen/logrus"
)
// Create a new instance of the logger. You can have any number of instances.varlog = logrus.New()
funcmain() {
// The API for setting attributes is a little different than the package level// exported logger. See Godoc.
log.Out = os.Stdout// You could set this to any `io.Writer` such as a file// file, err := os.OpenFile("logrus.log", os.O_CREATE|os.O_WRONLY, 0666)// if err == nil {// log.Out = file// } else {// log.Info("Failed to log to file, using default stderr")// }
log.WithFields(logrus.Fields{
"animal": "walrus",
"size": 10,
}).Info("A group of walrus emerges from the ocean")
}

Fields

Logrus encourages careful, structured logging through logging fields instead of
long, unparseable error messages. For example, instead of: log.Fatalf("Failed to send event %s to topic %s with key %d"), you should log the much more
discoverable:

We've found this API forces you to think about logging in a way that produces
much more useful logging messages. We've been in countless situations where just
a single added field to a log statement that was already there would've saved us
hours. The WithFields call is optional.

In general, with Logrus using any of the printf-family functions should be
seen as a hint you should add a field, however, you can still use the
printf-family functions with Logrus.

Default Fields

Often it's helpful to have fields always attached to log statements in an
application or parts of one. For example, you may want to always log the
request_id and user_ip in the context of a request. Instead of writing
log.WithFields(log.Fields{"request_id": request_id, "user_ip": user_ip}) on
every line, you can create a logrus.Entry to pass around instead:

requestLogger:= log.WithFields(log.Fields{"request_id": request_id, "user_ip": user_ip})
requestLogger.Info("something happened on that request") # will log request_id and user_ip
requestLogger.Warn("something not great happened")

Hooks

You can add hooks for logging levels. For example to send errors to an exception
tracking service on Error, Fatal and Panic, info to StatsD or log to
multiple places simultaneously, e.g. syslog.

It may be useful to set log.Level = logrus.DebugLevel in a debug or verbose
environment if your application has that.

Entries

Besides the fields added with WithField or WithFields some fields are
automatically added to all logging events:

time. The timestamp when the entry was created.

msg. The logging message passed to {Info,Warn,Error,Fatal,Panic} after
the AddFields call. E.g. Failed to send event.

level. The logging level. E.g. info.

Environments

Logrus has no notion of environment.

If you wish for hooks and formatters to only be used in specific environments,
you should handle that yourself. For example, if your application has a global
variable Environment, which is a string representation of the environment you
could do:

This configuration is how logrus was intended to be used, but JSON in
production is mostly only useful if you do log aggregation with tools like
Splunk or Logstash.

Formatters

The built-in logging formatters are:

logrus.TextFormatter. Logs the event in colors if stdout is a tty, otherwise
without colors.

Note: to force colored output when there is no TTY, set the ForceColors
field to true. To force no colored output even if there is a TTY set the
DisableColors field to true. For Windows, see
github.com/mattn/go-colorable.

When colors are enabled, levels are truncated to 4 characters by default. To disable
truncation set the DisableLevelTruncation field to true.

You can define your formatter by implementing the Formatter interface,
requiring a Format method. Format takes an *Entry. entry.Data is a
Fields type (map[string]interface{}) with all your fields as well as the
default ones (see Entries section above):

Rotation

Log rotation is not provided with Logrus. Log rotation should be done by an
external program (like logrotate(8)) that can compress and delete old log
entries. It should not be a feature of the application-level logger.

Fatal handlers

Logrus can register one or more functions that will be called when any fatal
level message is logged. The registered handlers will be executed before
logrus performs a os.Exit(1). This behavior may be helpful if callers need
to gracefully shutdown. Unlike a panic("Something went wrong...") call which can be intercepted with a deferred recover a call to os.Exit(1) can not be intercepted.

Thread safety

By default, Logger is protected by a mutex for concurrent writes. The mutex is held when calling hooks and writing logs.
If you are sure such locking is not needed, you can call logger.SetNoLock() to disable the locking.

Situation when locking is not needed includes:

You have no hooks registered, or hooks calling is already thread-safe.

Writing to logger.Out is already thread-safe, for example:

logger.Out is protected by locks.

logger.Out is a os.File handler opened with O_APPEND flag, and every write is smaller than 4k. (This allow multi-thread/multi-process writing)